The present invention relates to arrangements for increasing contrast when reproducing originals containing line elements dot by dot and line by line of the kind, comprising an opto-electronic scanning member having a picture-point aperture and a surrounding-area aperture to allow a picture signal and a surrounding-area signal to be obtained, a difference stage to which the picture and surrounding-area signals are applied to generate a correcting signal, and means to act on the correcting signal before it is superimposed on the picture signal. Hereinafter such arrangements will be referred to as "of the kind described".
Arrangements according to the invention may be applied to electronic reproducing apparatus such as engraving machines and scanners.
In such apparatus, an original is scanned point by point and line by line by an opto-electronic scanning member and the picture signal so obtained is corrected to suit the demands of the reproduction process. The corrected picture signal controls a reproducing member which is responsible for reproducing the original on a reproduction medium. The reproducing member is for example an electromagnetic engraving member for doing physical work on material or an exposing lamp for exposing films.
The original to be reproduced may contain contain continuous tone pictures and line elements belonging to characters and graphic representations.
The reproduction of very finely detailed line elements, such as occur for example in some foreign characters, presents considerable problems.
The original is produced photo-graphically and even at this stage the enlargement or copying causes a reduction in contrast in comparison with the original artwork, particularly in fine details, due to the unsharpness in the film layers.
It is for example difficult to produce black line elements whose density is the same as that of large black areas and nominally white areas lying between closely adjacent line elements become grey when they appear in the photographic original.
Thus, when the original is scanned in the reproducing apparatus, picture signals occur whose amplitudes deviate from the correct levels and the screen dots or tonal values which are reproduced from them become falsified.
Added to this is the fact that the resolution of the scanning member is restricted by light scatter and the aberrations of objective lenses. As a result a further reduction in contrast as compared with the original occurs in the reproduction process, which the human eye perceives as unsharpness in the reproduction.
To restore contrast or sharpness in the reproduction, it is known, from U.S. Pat. No. 2,691,696 for example, to use a process similar to the unsharp masking used in photographic reproduction and to scan not only the actual picture point, using a picture joint aperture, but also its immediate vicinity (surrounding area) using a surrounding-area aperture. The additional surrounding-area signal which is obtained in this way, which is proportional to the mean brightness of the surrounding-area, is subtracted from the picture signal to produce a correcting signal. A selectable proportion of the correcting signal, whose amplitude is dependant on the jump in tonal value in the original, is superimposed on the picture signal.
This measure results in an increase in detail contrast or sharpness at jumps in tonal value since in the immediate vicinity of a jump in tonal value a dark detail is reproduced darker and a light detail lighter than at some distance from the jump in tonal value. In this case the range covered by the correction at a jump in tonal value can be varied by varying the diameter of the surrouding-area aperture.
This known arrangement is not suitable for raising contrast selectively for line elements. As already mentioned, the picture signal levels which occur when scanning line elements are low and as a result so too are the correcting signal amplitudes. If the desired effect is in fact to be achieved for line elements with the known arrangement, the correcting signal available must be superimposed on the picture signal at its full value.
However, since in the conventional arrangement the correcting signal acts in the same way in both continuous-tone and line areas of the original, when there are jumps in density in continuous tone areas unsightly seams occur, which must be considered very much of a disadvantage.
It is therefore an object of the invention to overcome or at least minimise the disadvantages referred to and to provide an arrangement for making a selective increase in contrast or sharpness for line elements, without continuous-tone areas being disfigured.